Heat-developable light-sensitive materials and processes for processing the same are described in, for example, Shashin Kogaku no Kiso (Fundamentals of Photographic Engineering), Corona Co., Ltd., Tokyo, pp. 553 to 555 (1979), Eizo Joho (Image Information), page 40, April (1978), Nebletts Handbook of Photography and Reprography, 7th Ed., Van Nostrand Reinhold Company, pp. 32-33, U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020, 3,457,075, British Patents Nos. 1,131,108, 1,167,777, and Research Disclosure, June 1978, pp. 9-15 (RD-17029).
These heat-developable light-sensitive materials are heat-developed by techniques such as a method of contacting the light-sensitive material with a hot plate of great capacity, a method of directly heating the light-sensitive material by irradiating with a laser, infrared rays, supersonic waves, high frequency waves, or the like, and a method of passing the light-sensitive material through heated gas. These conventional methods, however, have many disadvantages and are not sufficiently satisfactory for practical use. For example, the method utilizing a hot plate has disadvantages in that much time is taken in order to give uniform temperature to the hot plate and, therefore, the amount of electricity consumed is large and, further, if the light-sensitive material is not brought into close contact with the hot plate, uniform conduction of heat to the light-sensitive material is difficult. The method utilizing radiation such as from a laser has a disadvantage in that a compact system is difficult to design because the apparatus is inevitably increased in size as a result of the use of the radiation. A major disadvantage of the method utilizing gas is that much time is taken for the processing because the heat capacity of gas is small.
In order to overcome the above problems, several methods have been proposed. One of the methods is to use a heat generating electrically conductive layer in combination with a heat-developable light-sensitive layer. This method is described in, for example, U.S. patent application Ser. No. 206,368 now abandoned and Japanese Patent Application (OPI) No. 66442/73 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). In accordance with this method, however, it is generally difficult to obtain sharp images in short periods of time although a compact system can be realized. In conventionally used electrically heat generating layers (i.e., layers generating heat by charging thereto with electric current) of cellulose based polymers, for example, the electric resistance does not greatly change with temperature and, therefore, the layers are easily overheated when a constant voltage is applied thereto. For this reason, the conventional electrically heat generating layers cannot be heated to a given temperature rapidly and in a stabilized manner. In order to avoid this overheating, it is necessary to employ a heating system having a control function to detect the actual temperature of the layer and to change the voltage according to the difference between the established temperature and the actual temperature. The conventional electrically heat generating layers have additional disadvantages. One of the disadvantages in respect of performance is that an unevenness (i.e., ununiformity) in the electric resistance over the light-sensitive material produces an unevenness in the heating temperature. This will lead to uneven development of the light-sensitive material. Another disadvantage is that explosionproof coating equipment is needed because binder polymers soluble only in organic solvents are used; that is, the conventional electrically heat generating layers are unsatisfactory from the viewpoints of safety and workability and also from an economical standpoint. Further disadvantage is that the light-sensitive material having such an electrically heat generating layer is not well balanced in curling because the light-sensitive material usually comprises a support, a light-sensitive layer containing gelatin as a binder provided on one side of the support and an electrically conductive layer made mainly of an organic solvent-soluble binder provided on the other side of the support; that is, the two layers provided on both sides of the support are made of different polymers.
As another method to overcome the above described problems, various positive types of electrically conductive layers (i.e., electrically conductive layers increasing their electric resistance as the temperature is increased) have been developed as plate shaped heaters (see, for example, Japanese Patent Application (OPI) Nos. 82734/74, 82735/74, 13991/76, 39742/76, 39743/76 and 87694/77). Since, however, these plate shaped heaters are intended to be used repeatedly, the unevenness in the heating temperature is accounted of little unless water-insoluble binders such as binders less influenced by the circumstances and binders dissolved in organic solvents or latexes are used. Thus, this technology cannot be applied to heat-developable light-sensitive materials as it is.
In order to overcome the above described problems, a light-sensitive material suitable for heat development by passing electric current has already been proposed by the present inventors (see Japanese Patent Application No. 229377/83). In this light-sensitive material, however, it has been desired to further improve the electrically conductive layer.